As a slot-in type disk conveying method, there are a roller method and an arm method. In order to provide a constant clearance with respect to a traverse (spindle motor) when a disk is conveyed, a traverse unit is vertically moved in both methods. For vertically moving the traverse unit, there are a vertically parallel moving method and a so-called one-side inclining moving method. In the latter method, only one side is rotated and vertically moved around a rotation center shaft provided on the side of a disk insertion opening. The traverse unit is fixed, and a clamper is vertically moved in some cases. In such a mechanism, as a method for mounting a disk on a rotation stage of a spindle motor, there are a clamper method, a pawl chuck method and a ball chuck method. In recent years, it is required to reduce a disk apparatus in size and thickness in line with the downsizing trends of personal computers. In order to reduce a slot-in type disk apparatus in thickness, the clamper method has a small space and its structure is difficult. Therefore, there are used the pawl chuck method or the ball chuck method in which a disk is pushed against a convex portion provided on a lid of a disk apparatus, a disk is held by a pawl or a ball provided on a rotation portion of the spindle motor and thus, the disk is mounted (e.g., patent document 1).
[Patent document 1] Japanese Patent Application Laid-open No. 2002-352497 (FIGS. 12 to 14)
In the so-called one-side inclining moving method in which only one side is rotated and vertically moved around the rotation center shaft provided on the side of the disk insertion opening, a traverse unit having a spindle motor is vertically moved, a disk is pushed against a convex portion provided on a lid of a disk apparatus, and the disk is held and mounted by a pawl or a ball provided on a rotating portion of the spindle motor. At that time, a rotation stage of the spindle motor is inclined.
FIG. 6 is a sectional view of an essential portion of a disk apparatus according to the conventional technique for explaining this state. FIG. 7 is a sectional view of an essential portion of a lid of the disk apparatus. FIG. 7 shows a mounting operation state of a disk 14 on a spindle motor 1. After a disk is inserted, a traverse 2 is displaced in a direction in which a spindle motor 1 approaches a lid 400 around a rotation support shaft X on the front side. When the spindle motor 1 is moved in a direction in which the spindle motor 1 most approaches the lid 400, as shown in the drawing, the disk 14 abuts against the lid 400, and the disk 14 is pressed by the spindle motor 1 and the lid 400. A pawl or a ball (not shown) provided on a hub 1a of the spindle motor 1 is fitted into a center hole of the disk 14 by this pressing force, and the disk 14 is mounted on the hub 1a and the rotation stage 1b. 
However, a convex portion 401 provided on the lid 400 projects in parallel with the lid 400, and the spindle motor 1 is displaced in the direction in which the spindle motor 1 most approaches the lid 400 around the rotation support shaft X on the front side. Therefore, when the traverse 2 most approaches the lid 400, as shown in the drawing, an upper surface of the rotation stage 1b which receives a disk of the spindle motor 1 is inclined with respect to a tip end surface of the convex portion 401 through an angle A°. Therefore, only a portion of a tip end of the convex portion 401 abuts against the disk, a gap B is formed on the other portion, the disk 14 can not be pressed uniformly, a disk is not reliably fitted to the hub 1a and the rotation stage 1b, and a so-called clamp miss occurs.
Hence, it is an object of the present invention to provide a thin disk apparatus capable of reliably mounting a disk on a rotation stage of a spindle motor.